John Glenn's historic flight aboard Friendship 7 took place more than 39 years ago. I was his backup pilot. Rightly or wrongly, Deke Slayton was ruled ineligible for the follow-up flight because of a minor heart condition. Rightly or wrongly, I was named to take his place.

All manners of hard feelings stemmed from these two unpopular decisions, which I did not make and over which I had no control. I took the flight, named my capsule Aurora 7 and trained hard for the six weeks I had. The flight plan, originally Deke's, called for a number of radical space maneuvers, more photography, more observation and some experiments, all of which I accomplished, in addition to returning safely to earth with the capsule unharmed. These facts alone should be enough to vindicate the flight of Aurora 7.

But there was even more: the system failures I encountered during the flight would have resulted in loss of the capsule and total mission failure had a man not been aboard. My postflight debriefings and reports led, in turn, to important changes in capsule design and future flight plans. These too I consider major contributions to our knowledge about spaceflight and to the successes that Wally Schirra and Gordo Cooper met with in their own subsequent flights.

Chris Kraft and I have always been at odds about my flight. Yet in his review of ''Flight,'' Henry Cooper seems unaware of the dispute. Chris's style, he says admiringly, succeeds in ''pulling no punches,'' as though this promises a candid and true book, when it could instead be merely vindictive and skewed. A question lingers in this rattled old septuagenarian brain: why is Chris still in the ring throwing punches?

Finally, the reviewer notes with interest Chris's early, stark realization that ''things happened so fast in rocketry that an astronaut couldn't do anything'' without help from the ground. A debatable proposition. In any event, Chris never acknowledged that the reverse also holds true: in space things happen so fast that only the pilot knows what to do, and

even ground control can't help. Maybe that's why he is still fuming after all these years.